Infusible unidirectional fabric

ABSTRACT

An infusible, unidirectional fabric containing a plurality of unidirectional fibers spaced uniformly in the unidirectional fabric, a plurality of bridges, and a plurality of void spaces between the unidirectional fibers. Each bridge is connected to at least 2 unidirectional fibers and at least 70% by number of fibers have at least one bridge connected thereto forming a bridged network of unidirectional fibers. The void spaces are interconnected and the fabric has a volume fraction of voids of between about 8 and 70%, a volume fraction of fibers of between about 35 and 85%, and at least 50% by number of the bridges have a bridge width minimum less than about 2 millimeters.

RELATED APPLICATIONS

This application claims priority to U.S. provisional application61/730,677, filed Nov. 28, 2012, the contents of which are incorporatedherein by reference in their entirety.

FIELD OF THE INVENTION

The present invention generally relates to infusible, unidirectionalfabrics.

BACKGROUND

The development of more structurally efficient composite materialsenables higher performance and more cost competitive solutions across arange of markets which use these materials. The traditional forms usedfor introducing dry fibers such as glass roving or carbon tow into acomposite system are fabrics such as woven fabrics (having crimping) ormulti-axial knit fabrics (with minimal crimping). These fabric formstypically impart performance penalties on the final composite system.

Composites reinforced with woven fabrics are known to exhibit lowermodulus and strength due to the extensive fiber crimping which occurs asopposing direction fibers cross over each other. In the case ofmulti-axial knits, the layers of reinforcing fibers do notinterpenetrate each other. The knitting process employs a stitch yarnwhich is looped around the reinforcing fibers tying the fibers togetherand providing stability to the fabric. The stitch yarn creates localdeviations in yarn direction and imparts a subtle waviness along thefiber axis direction. The stitch yarns typically create a separation orgap between rovings or tows within a fabric and between layers of fabricwhile not offering any improvement in mechanical properties.Furthermore, the gaps created by the presence of the stitch yarns reducethe maximum achievable fiber volume fraction of a composite made withsuch reinforcement. Finally, the fiber waviness negatively impactsseveral structural properties of composites reinforced with such systemssuch as tensile modulus and compression strength.

There is an opportunity to develop infusible composite fabrics thatoffer high fiber volume fractions, a high degree of fiber alignment andstraightness with excellent fiber distribution uniformity. These fabricsshould be convertible into composite parts through common compositemolding operations such as vacuum infusion or resin transfer molding.These characteristics enable superior structural properties whilepreserving the cost advantages of well-established resin infusionprocessing. A new approach for delivering a composite preform with theseattributes is described.

BRIEF SUMMARY

An infusible, unidirectional fabric containing a plurality ofunidirectional fibers spaced uniformly in the unidirectional fabric, aplurality of bridges, and a plurality of void spaces between theunidirectional fibers. Each bridge is connected to at least 2unidirectional fibers and at least 70% by number of fibers have at leastone bridge connected thereto forming a bridged network of unidirectionalfibers. The void spaces are interconnected and the fabric has a volumefraction of voids of between about 8 and 70%, a volume fraction offibers of between about 35 and 85%, and at least 50% by number of thebridges have a bridge width minimum less than about 2 millimeters.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional illustrative view of one embodiment of aninfusible, unidirectional fabric.

FIG. 2 is a cross-sectional illustrative view of one embodiment of aninfused, unidirectional composite.

FIG. 3 is a photographic, cross-sectional image of one embodiment of aninfusible, unidirectional fabric.

FIG. 4 is a photographic, cross-sectional image of one embodiment of aninfused, unidirectional composite.

FIG. 5A is a micrograph of one embodiment of the unidirectional fabricalong the length of the fibers showing bridges.

FIG. 5B is an illustration of FIG. 5A.

FIG. 6 illustrates the method for determining uniformly spaced fibers.

FIG. 7 is an illustrative view of a wind turbine.

FIGS. 8-12 are illustrative views of a turbine blade.

DETAILED DESCRIPTION

FIG. 1 is an illustration of one embodiment of an infusible,unidirectional fabric 10. The infusible, unidirectional fabric 10contains a bridged network of unidirectional fibers 100 which contain aplurality of fibers 110 and a plurality of bridges 200. The infusible,unidirectional fabric 10 also contains void spaces 120 surrounding thefibers 110. The bridged network of unidirectional fibers 100 has anupper inner surface 10 a and a lower inner surface 10 b. The upper andlower inner surfaces are defined as the boundaries which contain betweenthem essentially all of the fibers within the bridged network ofunidirectional fibers excluding any unique features occurring only nearthe edges or edge effects. Edge effects might include a polymer richskin or a region of non-uniform fiber spacing. FIG. 3 is a micrographimage of one embodiment of the infusible, unidirectional fabric.

The unidirectional fabric may be any suitable width and in any suitableshape. In some embodiments where the width of the fabric is smaller,typically between about 2 and 300 mm, the fabric may be referred to as aunidirectional tape or fabric band.

Once the infusible, unidirectional fabric 10 is infused with resin andcured, an infused, unidirectional composite 400 illustrated in FIG. 2 isformed. In the infused, unidirectional composite, the resin 300 coatsand at least partially infuses into the bridged network ofunidirectional fibers 100 and cures at least partially filling the voidspace 120 in the bridged network of unidirectional fibers 100. Thisforms the infused, unidirectional composite 400 containing bridgednetwork of unidirectional fibers 100 which contains fibers 110, bridges200, and resin 300. FIG. 4 is a micrograph image of one embodiment of aninfused, unidirectional composite.

A fabric with the above described structure will be infusible in vacuumassisted resin transfer molding (also called vacuum assisted resininfusion) process. The word “infusible”, in this invention, refers tofabrics having the following characteristics: The fabrics can be used tomake fiber reinforced polymer composites having a thickness greater than2 mm by using a standard vacuum assisted resin transfer molding (alsocalled vacuum assisted resin infusion) method and low viscosity infusiongrade thermoset resin. The infusion process has a typical processingtime scale ranging from minutes to hours. Preferably, the finishedcomposite with the infusible fabric typically has a void content asmeasured by a standard test such as ASTM D2734 of less than 5%, morepreferably less than 2%.

A simple method to predict whether a fabric is infusible or not can bedescribed as follows. Several water droplets with 0.01% water solublecolor dye (for example, Acid Blue 9) are dropped on the surface offabric by using a 5 mL transfer pipette. The time duration required forthe droplets to completely infuse into the fabric is used as anindication of infusibility. By definition, in this method, “completelyinfuse into the fabric” means that more than 99% by mass of the waterfrom the original droplet has been absorbed between the upper innersurface and lower inner surface of the fabric. By definition, a fabricis considered “infusible” in this invention if the average water dropletinfusion time is less than 1 minute. This method is a method to indicatethat the fabric is “infusible”, if the average water droplet infusiontime is longer than 1 minute, it is not necessary to mean that thefabric is not resin infusible due to the hydrophobic nature of mostthermoset resin. This test method may not be accurate if there iscoating strong hydrophobic tendencies on the fabric.

Preferably, the infusible, unidirectional fabric 10 is self-supporting.“Self-supporting”, in this invention, means that the fabric isdimensionally stable, and the fibers in the fabric will not fall apartdue to their own weight under gravity. The fabric has a well-definedwidth and thickness. Additional components may be attached to the fabricbut are not required. Preferably, additional stabilizing means such asstitching, scrims, films, and the like are not needed to handle andconvey the infusible, unidirectional fabric 10.

Within the infusible, unidirectional fabric 10, the void spaces areinterconnected and the fabric has a void fraction of preferably betweenabout 8 and 70%, more preferably between about 10 and 70%. Theinfusible, unidirectional self-supporting fabric preferably has a fibervolume fraction between 35% and 85%, preferably between 45% and 80%,more preferably between 50% and 80%. A fiber volume fraction less thanabout 30% may make the fiber reinforcement less practical as a compositereinforcement. A fiber volume fraction greater than 85% could havenegative consequences as it may slow down the resin infusion process,reduce the mechanical properties perpendicular to the fiber direction,or reduce the fatigue durability of the composite. If the void spacesare not interconnected, there may be to too few channels for resininfusion. If there is not enough void content in the fabric, resininfusion may be very slow and difficult.

The fiber volume fraction can be measured using a first method (forfabric made by inorganic fibers) where one measures the total mass (m₀),thickness (D), width (W), and length (L) of given piece of fabric, andthen calculates the total volume (V₀) of the given piece of fabric byV₀=D×W×L. Next, the sample (piece of fabric) is placed in an oven,heated at 700° C. for 4 hours to burn off all organic content in thefabric, and the mass of inorganic component is measured (mass (m_(f)))after this burn off step. The fiber volume fraction (V_(f)%) iscalculated by V_(f)%=(m_(f)/ρ_(f))/V₀, where ρ_(f) is the density of thematerial which made the fiber. ρ can be measure by any suitable densitymeasurement methods, or obtained from technical data sheet of the fibermaterial. The method works only when there is no or very small amount(less than 1%) other inorganic component such (for example, silicananoparticles) in the fabric.

Another method to measure the void content in the fabric can bedescribed as the following: use the infusible unidirectional fabric tomake a fiber reinforced composite material by using vacuum assistedresin infusion method (detail description of this method is described inthe Example section below) and do a SEM or optical imaging to a typicalcross section of the composite, where the cross section is perpendicularto the fiber direction. The void content can be calculated by measuringthe total cross section area of infused resin, divided by the totalcross section area of the composite. To help identifying the infusedresin area, about 0.01% to 0.1% by weight of color dye or fluorescentdye can be added into the resin before resin infusion.

The infusible, unidirectional self-supporting fabric also comprisespolymer bridges, where the volume ratio of polymer bridges to fibers isbetween 1:370 and 1:2, more preferably between 1:40 and 1:4, morepreferably between 1:12 and 1:4. The polymer bridges are a main sourceof support to the fabric structure and help prevent fibers fall apartdue to gravity. The overall polymer bridge structure will not be strongenough to support the fabric structure if there are too few polymerbridges in the fabric. If there is too much polymer in the fabric, theremay not be enough void space for resin infusion. The total volume ofpolymer bridges can be calculated by knowing how much mass (m_(p))polymer bridge material(s) have been added into the fabric duringmanufacturing, or using a burn off test (Described in the first methodabove for measuring fiber volume) to estimate the mass of polymerbridges by m_(p)=(m₀−m_(f)). The volume of polymer bridges (V_(p)) iscalculated by V_(p)=m_(p)/ρ_(p), where ρ_(p) is the density of polymermaterial.

The infusible, unidirectional fabric 10 (and composite 400) contains abridging polymer which forms bridges 200 between and connected to atleast a portion of the fibers 110. This is shown in both FIGS. 1 and 2.Preferably, each bridge is connected to at least 2 unidirectional fibersforming bridged fibers. In one embodiment, at least 70% by number, atleast 80% by number, or at least essentially all of the fibers 110 arebridged to at least one other fiber 110 somewhere along the length ofthe fiber. “Essentially all”, this this context means that enough of thefibers are attached such that there are no loose fibers, therefore thefabric acts as a unit not like a yarn. In another embodiment, at leastabout 90% by number of the fibers 110 are bridged to at least one otherfiber 110 somewhere along the length of the fiber. As the % connected bynumber of fibers is anywhere along the length of the fiber, in a typicalsingle cross-section, fewer connections will be seen.

Therefore, in a given cross-section, preferably between about 10 and100% by number of fibers contain bridges to one or more fibers withinthe bridged network of unidirectional fibers 100 (composite 400). Inanother embodiment, between about 15 and 100% by number of fibers in agiven cross-section contain bridges to one or more fibers, morepreferably between about 50 and 100%, more preferably between about 60and 100% more preferably between about 75 and 100% by number of fibersin a given cross-section.

Within the bridged network of unidirectional fibers 100, there are aplurality of bridges 200 between and connected to at least a portion offibers 110. The bridging between fibers 110 helps control the positionof the fibers 110 relative to other fibers and the fabric. The bridgingattaches the fibers together and creates a stable fabric form. Thesebridges are connected and adhered to the surface of the fibers 110. Abridging polymer that extends between at least two fibers 110 but is notattached to at least two fibers 110 is not a bridge as defined in thisapplication. The bridging increases the interaction between fibers 110while still allowing resin to flow between and around the fibers 110.The bridging polymer preferably has an elasticity which is characterizedas elongation at break at least about 50%, more preferably higher than100%, and more preferably higher than 300%. The elasticity of thebridges helps the fabric remain flexible (able to conform to curved moldshapes) and helps the bridges survive bending or folding of the fabric.

The bridging polymer may be physically or chemically bonded (throughthere may be in some embodiments a thin layer between anchoring surfaceand fiber surface, for example, a coating layer or sizing) to thesurface of the fiber 110 through interactions including but not limitedto hydrogen bonding, van der Waals interactions, ionic interactions,electrostatic interactions, mechanical interlocking, or a portion of theanchoring surface may chemically react with the surface of the fiber 110to form covalent bonds between the fiber and the anchoring surface. Theanchoring surface may be physically or chemically bonded to a coating orsizing that was previously applied to the fiber, through interactionsincluding hydrogen bonding, van der Waals interactions, ionicinteractions, electrostatic interactions, or a portion of the anchoringsurface may chemically react with the coating or sizing on the surfaceof the fiber to form covalent bonds between the coating or sizing on thefiber surface and the anchoring surface. If the fiber or coating orsizing on the fiber is porous or if the precursors to the bridge candiffuse or penetrate into the surface of the fiber, then the anchoringsurface may interpenetrate with the fiber surface on a nanometer ormicrometer length scale. It is important that the bridging polymer hasgood adhesion to fiber surface, because all the fibers in theunidirectional fabric structure are held together by the bridges.

In one embodiment, at least a number of the bridges contain a widthgradient, where the width of the bridge is greatest at the anchoringsurface and decreases in a gradient away from the anchoring surface. Thegreater width at the anchoring surface helps increase the strength ofthe adhesion between the bridge and the fiber, and a narrower width awayfrom the anchoring surface leaves more void space in the fabric 10 forresin infusion. An optimized system is preferred which has sufficientstrength for maintaining fabric integrity during handling whileminimizing the time required to infuse the structure with resin.

Additionally, preferably at least 50% of the bridges have a bridge widthminimum narrower than 2 mm, more preferably narrower than 0.5 mm, morepreferably narrower than 0.2 mm. The bridge width minimum is defined asthe minimum width of the bridge (in the direction of fiber length) fromsurface of the first fiber to the surface of the second connected fiber.In one embodiment, the bridges typically have approximately the samewidth along the fiber direction from the surface of one fiber to theconnected fiber. In this case, the bridge width is approximatelyconstant in the bridge. In another embodiment, the bridge is wider wherethe bridge attaches to the fibers and is the narrowest (and has theminimum width) between the two fibers.

The width of the bridges in fiber direction can be measured by opticalmicroscopic image or SEM image. In this measurement, dry fabric (beforeresin infusion) is preferred to be used to take images. The images aretaken from the cross section which is parallel to fiber direction. FIGS.5A and 5B show some typical bridges in the unidirectional fabric andcomposites. FIG. 5A is a micrograph image and FIG. 5B is an illustrationof the photograph of FIG. 5A. FIGS. 5A and 5B show some typical bridges.If the width of the bridges in the fiber direction is too wide (andtherefore the bridge width minimum is too large), the resin is less ableto infuse through the fabric in the thickness direction.

In one embodiment, the bridges 200 preferably form between about 0.1 and60% of the effective cross-sectional area of the infusible,unidirectional fabric 10 (and infused, unidirectional composite 400). Inanother embodiment, the bridges 200 form between about 0.1 and 30% ofthe effective cross-sectional area of the fabric and composite, morepreferably between about 0.3% and 10%, more preferably between about0.5% and 5%. “Effective cross-sectional area”, in this application, ismeasured by taking a cross-sectional image of the fabric and calculatingthe area of bridge. If the cross-sectional area of bridges is less thanabout 0.1%, there may not be enough bridges to enhance the mechanicalproperties of the composite. If the cross-sectional area of bridges islarger than 30%, there may not be enough porosity in the fabric forresin infusion leading to lower performance due to dry spots or voids inthe composite systems.

Where bridging occurs in the fabric 10 depends on a number of factorsincluding but not limited to the type of bridging polymer, solvent, filmforming preventing agent, surface chemistry of fiber, separationdistance between fibers, coating process conditions, drying conditions,post mechanical treatment during and after drying. The time required forbridging to occur also depends on concentration of bridging polymer,concentration of co-stabilizer, concentration of surfactant, surfacechemistry of fiber, initial size of dispersed phase in the emulsion,temperature, solidification time of the bridging polymer, separationdistance between adjacent fibers, and coating process conditions,

In one embodiment, the bridging polymer forms between about 1% and 20%by weight of the infusible, unidirectional fabric. In anotherembodiment, the cross sectional area of the fibers is between 30% and80% of the total cross sectional area of the fabric, and the ratio bycross sectional area of polymer:void is between 1:0.5 and 1:93.

The anchoring surfaces of bridges cover less than 100% of the fibersurfaces (this includes all of the surface area of the fiber). Theuncovered fiber surfaces can bond to the resin directly in compositesand increase the interaction between fibers and infused resin incomposite. In one embodiment, the anchoring surfaces of bridges coverabout 10% to 99% of the fiber surface. Preferably the anchoring surfacesof bridges cover about 30% to 90% of the fiber surface.

The bridges in the infusible, unidirectional fabric are formed from abridging polymer including but not limited to thermoset resin,thermoplastic resin, ionomer, dendrimer, and mixtures thereof. Thermosetresins, such as epoxy, polyurethane, acrylic resin, rubbers, andphenolic, are liquid resins which harden by a process of chemicalcuring, or cross-linking, which takes place during the coating process.Thermoplastic resins, such as polyethylene, polypropylene, polyolefincopolymer elastomer, thermoplastic polyurethane, polyvinyl alcohol(PVA), PET and PEEK, are liquefied by the application of heat prior tocoating and re-harden as they cool within the fabric. Preferably, thebridging polymer has good adhesion on fiber surface. Preferably, thebridging polymer (or the polymer in organic solvent solution, or thechemicals that form polymer during process) can be uniformly dispersedin water before coating. In one embodiment, the bridging polymer isethylene vinyl acetate (EVA) copolymer, styrene butadiene rubber (SBR),water borne polyurethane, polyolefin elastomer (POE), or a mixturethereof. SBR and polyurethane are preferred due to its moderate cost,good mechanical properties, and good adhesion to fibers.

In one embodiment, the polymer bridges are formed beginning with apolymer in water dispersion or polymer water solution. SBR latex orwater borne polyurethane are preferred due to its moderate cost, goodmechanical properties, good adhesion to fibers. Film-forming preventingagents are preferred to be added in to the polymer water dispersion orpolymer water solution, because the film-forming preventing agents cancreate void space and channels between fibers by preventing the polymerforming continuous film.

In one embodiment, the film-forming preventing agent is solid or liquidparticles which can be dispersed or dissolved in the polymer in waterdispersion or polymer water solution. This type of film formingpreventing agent will be removed from the fabric after the polymersolidified. Silica particles are one of the examples. In one embodiment,the film-forming preventing agent is water soluble material, which canphase separate from the polymer and form continuous phase during waterevaporation. One requirement of the water soluble materials is that theydon't make the polymer in water dispersion or polymer water solutionunstable. In one embodiment, sugar or other water soluble non-ionicmaterials are preferred. In another preferred embodiment, glycerin orpropylene carbonate is used as a film forming preventing agent to createthe void space. After water evaporation and polymer solidified, the filmforming preventing agent rich phase will be removed from the fabric,leaving voids and channels in the fabric.

In one embodiment, the film-forming preventing agents are a combinationof blowing agents, and frothing agents or foaming agents. The blowingagent can be any suitable material that can create bubbles duringcoating process. In one embodiment, the blowing agent is water. Watercan quickly evaporate under heat and creates bubbles. In anotherembodiment, the blowing agent is carbon dioxide that has dissolved inwater. In another embodiment, the blowing agent is low boiling pointorganic liquid. In another embodiment, the blowing agent can chemicallydecompose and release gas under heat. This type of blowing agentincludes but not limit to NaHCO₃, azodicarbonamide, and p-p′-oxbis(benzensulfonyl hydrazide). The frothing agents or foaming agentsinclude but not limited to ionic surfactant such as sodium dodecylsulfate (SDS), sodium dodecylbenzenesulfonate (NaDDBS), or non-ionicblock copolymer such as ethylene oxide and propylene oxide copolymer.One example of the block copolymer is Pluronic® from BASF. A gellingagent is also preferred to be added to stabilize the polymer foam. Thegelling agent includes but not limited to acacia, alginic acid,bentonite, carbomers, carboxymethylcellulose. ethylcellulose, gelatin,hydroxyethylcellulose, hydroxypropyl cellulose, magnesium aluminumsilicate (Veegum®), methylcellulose, Pluronics®, polyvinyl alcohol,sodium alginate, tragacanth, and xanthan gum. A gelling agent with lowercritical solution temperature (LCST) is preferred because it is solublein cold water and gels in hot water. One example of the gelling agentwith LCST characteristic is Pluronics® F-127.

In one embodiment, sugar is used as the film-forming preventing agent.The polymer solid content is between about 1% and 60% of the polymer inwater dispersion or polymer in water solution. More preferably thepolymer solid content is between about 3% and 20%. The sugar to polymersolid content ratio by weight is between about 0.5:1 and 10:1, morepreferably between 1:1 and 5:1. Too little sugar cannot prevent polymerforming films and cannot create enough void space and channels in thefabric; Too much sugar makes the polymer bridges weak.

In one embodiment, glycerin is used as the film-forming preventingagent. The polymer solid content is between about 1% and 60% of thepolymer in water dispersion or polymer in water solution. Morepreferably the polymer solid content is between about 3% and 20%. Theglycerin to polymer solid content ratio by weight is between about 0.5:1and 20:1, more preferably between 1:1 and 10:1. Too little glycerincannot prevent polymer forming films and cannot create enough void spaceand channels in the fabric; too much glycerin makes polymer dispersionunstable and the polymer bridges are weak.

In another embodiment, foaming agents and gelling agents are used as thefilm-forming preventing agent. The polymer solid content is betweenabout 1% and 60% of the polymer in water dispersion or polymer in watersolution. More preferably the polymer solid content is between about 3%and 20%. The frothing agent is between about 0.1% and 20% of the totalweight, more preferably between about 1% and 10% of the total weight.The gelling agent is between about 0.1% and 40% of the total weight,more preferably between 1% and 10% of the total weight. In oneembodiment, Pluronics® F-127 is used as a foaming agent and also agelling agent, preferably between 1% and 15% by weight in the coatingmixture, more preferably between 3% and 10% by weight in the coatingmixture.

In one embodiment, the bridging polymer and the resin 300 have differentchemical compositions. Having a different chemical composition, in thisapplication, means that materials having a different molecularcomposition or having the same chemicals at different ratios orconcentrations. Having different chemical compositions may be able tohelp redistribute stress in composites. In another embodiment, thebridging polymer and the resin 300 have the same chemical compositions.Having the same compositions may make the infusing resin wet the fabricmore easily.

The bridged network of unidirectional fibers 100 may be any suitablefibers for the end product. “Unidirectional fibers”, in this applicationmeans that the majority of fibers aligned in one direction with the axisalong the length of the fibers being generally parallel. The composite400 may contain a plurality of fibers in a bundle (the bundles may bepart of a textile layer including but not limited to a woven textile,non-woven textile (such as a chopped strand mat), bonded textile, knittextile, a unidirectional textile, and a sheet of strands.) In oneembodiment, the bridged network of unidirectional fibers 100 are formedinto unidirectional strands such as rovings and may be held together bybonding, knitting a securing yarn across the rovings, or weaving asecuring yarn across the rovings. In the case of woven, knit, warpknit/weft insertion, non-woven, or bonded the textile can have fibersthat are disposed in a multi- (bi- or tri- or quadri-) axial direction.In one embodiment, the bridged network of unidirectional fibers 100contains an average of at least about 2 fibers, more preferably at leastabout 20 fibers. The fibers 110 within the fabric 10 generally arealigned and parallel, meaning that the axes along the lengths of thefibers 110 are generally aligned and parallel. Each fiber has a fibersurface defined to be the outer surface of the fiber and a fiberdiameter.

Preferably, the infusible, unidirectional fabric 10 containsunidirectional fibers 110 that are spaced uniformly in theunidirectional fabric 10. “Spaced uniformly” or “uniformly spaced”, inthis application, means that in a typical fabric cross section, withinthe bridged network of unidirectional fibers, there is no clear boundaryof any fiber bundle, yarn, roving, or tow.

For the purpose of this invention, fiber distribution uniformity can bemeasured by the following method. A typical cross section image of theunidirectional fabric or composite made thereof is prepared by standardmicroscopy mounting and imaging techniques. Unidirectional fabrics aretypically encapsulated in a polymer such as mounting epoxy and cut witha diamond wafer saw orthogonal to the fiber direction through thesample. Composites can often be sectioned without requiring mountingbecause the fibers are already stabilized by the composite matrixpolymer.

After sectioning, the surface of the cross section to be viewed isground and polished to enable unobstructed viewing of the sample throughoptical or electron microscopy. The polishing process is repeated untilthe contrast between fiber and matrix in the images at the targetresolution is sufficient to compute the fiber area fraction within thecross section. The perimeter of each fiber should be clearlydistinguishable.

For measuring fiber distribution uniformity via the fiber area ratiomethod described herein, the image must be of a sufficient size scale toencompass the entire thickness of at least one layer of the fabric. Anexample image of a composite reinforced with two layers ofunidirectional fabric, 501 and 501, comprising glass fibers is shown inFIG. 6. Within the layer to be analyzed, 501, the upper inner surface,10 a, and lower inner surface, 10 b, are located. The distance betweenthe upper inner surface and the lower inner surface is defined as thebulk thickness, t_(b).

Within the unidirectional region of the cross section image locatedbetween the upper inner surface and the lower inner surface, a grid ofsquares is overlaid onto the image, 510. The grid contains a squarepattern of non-overlapping connected squares which share edges andcorners, 520. Each square in the grid has sides of length t_(b)/2. Theimage must be of sufficient size to contain at least four such squares.The number of grid squares should be the maximum possible within thecross section area of the fabric where each sub-region remains fullywithin the fabric cross section. Each area of the image within theborders of each square, 521, is defined as a sub-region.

For each sub-region, the fiber area fraction is computed. The fiber areafraction for a sub-region is the ratio of the area within the sub-regionthat is occupied by fiber divided by the total area of the sub-region.This calculation is readily done by standard image processing algorithmsbased on the image contrast or color difference between the fiber andthe matrix region.

After all sub-regions of the typical cross section image have beenanalyzed, the overall average fiber area ratio can be computed. Auniform distribution is defined as one in which at least 85% of thesub-regions have a fiber area ratio value that falls within the rangedefined by ±15% of the overall average fiber area ratio. More preferablythe distribution is characterized by at least 95% of the sub-regionshaving a fiber area ratio value within the range defined by ±15% of theoverall average fiber area ratio. Most preferably, the distribution ischaracterized by at least 98% of the sub-regions having a fiber arearatio value within the range defined by ±15% of the overall averagefiber area ratio.

In some embodiments, a composite contains more than one fabric or agroup of fabrics. The same definition of “uniform distribution” can beapplied across cross section images containing regions of more than oneunidirectional fabric. A grid as described above is created within onelayer of the reinforcement, and then extended to encompass the entireunidirectional region of the composite less any residual area that doesnot fit within a full square defined by the grid. Fiber area ratios arecomputed within each sub-region. After all sub-regions of the typicalcross section image have been analyzed, the overall average fiber arearatio can be computed. A uniform distribution is defined as one in whichat least 85% of the sub-regions have a fiber area ratio value that fallswithin the range defined by ±15% of the overall average fiber arearatio. More preferably the distribution is characterized by at least 95%of the sub-regions having a fiber area ratio value within the rangedefined by ±15% of the overall average fiber area ratio. Mostpreferably, the distribution is characterized by at least 98% of thesub-regions having a fiber area ratio value within the range defined by±15% of the overall average fiber area ratio.

A composite comprising multiple layers of conventional unidirectionalfabrics may not be considered to have a uniform fiber distribution ifthe gaps created between the unidirectional yarns or rovings or towswithin the fabric or the gaps created between the layers of fabric arelarge enough to prohibit satisfying the criteria requiring at least 85%of the sub-regions have a fiber area ratio value that falls within therange defined by ±15% of the overall average fiber area ratio.

In this definition, a fabric having yarns or threads woven into theunidirectional fibers in the direction perpendicular to theunidirectional fibers would fall under the definition of spaceduniformly as typically the gap between rovings or bundles are about 4times of the fiber diameter. If a typical bundle of rovings is used,then the unidirectional fibers are grouped into bundles where the fibersin those bundles are held closer together and there is typically a spacebetween bundles where little to no fibers reside. Preferably, there areno additional fibers or yarns holding the unidirectional fiberstogether.

The strength and free-standing nature of the bridged network ofunidirectional fibers 100 is due mostly to the bridges 200. Preferably,the bridged fibers (containing no additional reinforcements besides thebridges) have enough tensile strength to be handled in a manufacturingprocess without any additional reinforcement fabrics or layers. Inanother embodiment, the bridged fibers have a tensile strength of atleast 200 Pa in the direction perpendicular to the length direction ofthe unidirectional fibers. In another embodiment, the bridged fibershave a tensile strength of at least 700 Pa, more preferably higher than10 kPa in the direction perpendicular to the length direction of theunidirectional fibers. The tensile strength of the fabric is measured bygripping two ends of a rectangular piece fabric in a tensile strengthtest machine (for example, Instron), while the tensile test direction isperpendicular to the unidirectional fiber direction. The fabric is thenstretched under a constant speed (typically about 1˜10 cm/minute). Thetensile strength is calculate by measuring the maximum tensile forcebefore fabric is broken, divided by the area of cross section of thefabric. In another embodiment, the fabric does not suffer significantstructural damage under a peel strength of 0.25 lbf/inch (0.44 N/cm) ina peel strength test between the fabric and an adhesive tape. In thistest, a piece of adhesive tape about 6˜10 inch long is adhered to thefabric surface in the fiber direction at room temperature, and the peelstrength between the tape and fabric is tested. The details of peelstrength test can be found in ASTM D5170. “Not suffer significantstructural damage”, in this invention, means that most fibers are stillkeeping their relative position in the fabric during peel strength test,and after the peel strength test, less than 20 fibers, preferably lessthan 10 fibers, more preferably zero fiber, are sticking on the adhesivetape. The fibers (if there is any) which are sticking on the adhesivetape are originally located on the surface of the fabric. This meansthat the interface between the tape and the fabric failed before thefabric cohesively failed. In one embodiment, the fabric contains noadditional stitching fibers, reinforcement layers, or reinforcementfabrics such as stitching yarns or scrims. Thus, the infusibleunidirectional fabric has enough strength to be used as a stand-alonefabric, for example allowing the fabric to be placed in the mold beforeinfusion with resin. Because additional stitching fibers, reinforcementlayers, or reinforcement fabrics usually creates gap or space with veryfew fibers, as a result, the fibers may not be spaced uniformly.

The fibers 110 may be any suitable fiber for the end use. “Fiber” usedherein is defined as an elongated body and includes yarns, tapeelements, and the like. The fiber may have any suitable cross-sectionsuch as circular, multi-lobal, square or rectangular (tape), and oval.The fibers may be monofilament or multifilament, staple or continuous,or a mixture thereof. Preferably, the fibers have a circularcross-section which due to packing limitations intrinsically providesthe void space needed to host the bridges. The fibers 110 can have anaverage length of at least about 3 millimeters. In another embodiment,the fiber length is at least about 100 times the fiber diameter. Inanother embodiment, the average fiber length is at least about 10centimeters. In another embodiment, the average fiber length is at leastabout 1 meter. Preferably, the fibers are continuous. The fiber lengthscan be sampled from a normal distribution or from a bi-, tri- ormulti-modal distribution depending on how the fabrics are constructed.The average lengths of fibers in each mode of the distribution can beselected from any of the fiber length ranges given in the aboveembodiments.

The fibers 110 can be formed from any type of fiberizable material knownto those skilled in the art including fiberizable inorganic materials,fiberizable organic materials and mixtures of any of the foregoing. Theinorganic and organic materials can be either man-made or naturallyoccurring materials. One skilled in the art will appreciate that thefiberizable inorganic and organic materials can also be polymericmaterials. As used herein, the term “polymeric material” means amaterial formed from macromolecules composed of long chains of atomsthat are linked together and that can become entangled in solution or inthe solid state. As used herein, the term “fiberizable” means a materialcapable of being formed into a generally continuous or staple filament,fiber, strand or yarn. In one embodiment, the fibers 110 are selectedfrom the group consisting of carbon, glass, aramid, boron, polyalkylene,quartz, polybenzimidazole, polyetheretherketone, basalt, polyphenylenesulfide, poly p-phenylene benzobisoaxazole, silicon carbide,phenolformaldehyde, phthalate and napthenoate, polyethylene. In anotherembodiment, the fibers are metal fibers such as steel, aluminum, orcopper.

Preferably, the fibers 110 are formed from an inorganic, fiberizableglass material. Fiberizable glass materials useful in the presentinvention include but are not limited to those prepared from fiberizableglass compositions such as S glass, S2 glass, E glass, R glass, H glass,A glass, AR glass, C glass, D glass, ECR glass, glass filament, stapleglass, T glass and zirconium oxide glass, and E-glass derivatives. Asused herein, “E-glass derivatives” means glass compositions that includeminor amounts of fluorine and/or boron and most preferably arefluorine-free and/or boron-free. Furthermore, as used herein, “minoramounts of fluorine” means less than 0.5 weight percent fluorine,preferably less than 0.1 weight percent fluorine, and “minor amounts ofboron” means less than 5 weight percent boron, preferably less than 2weight percent boron. Basalt and mineral wool are examples of otherfiberizable glass materials useful in the present invention. Preferredglass fibers are formed from E-glass or E-glass derivatives.

The glass fibers of the present invention can be formed in any suitablemethod known in the art, for forming glass fibers. For example, glassfibers can be formed in a direct-melt fiber forming operation or in anindirect, or marble-melt, fiber forming operation. In a direct-meltfiber forming operation, raw materials are combined, melted andhomogenized in a glass melting furnace. The molten glass moves from thefurnace to a forehearth and into fiber forming apparatuses where themolten glass is attenuated into continuous glass fibers. In amarble-melt glass forming operation, pieces or marbles of glass havingthe final desired glass composition are preformed and fed into a bushingwhere they are melted and attenuated into continuous glass fibers. If apre-melter is used, the marbles are fed first into the pre-melter,melted, and then the melted glass is fed into a fiber forming apparatuswhere the glass is attenuated to form continuous fibers. In the presentinvention, the glass fibers are preferably formed by the direct-meltfiber forming operation.

In one embodiment, when the fibers 110 are glass fibers, the fiberscontain a sizing. This sizing may facilitate processing of the glassfibers into textile layers and enhances fiber—polymer matrixinteraction. In another embodiment, the fibers 110 being glass fibers donot contain a sizing. The non-sizing surface may help to simplify thecoating process and give better control of polymer—fiber interaction.Fiberglass fibers typically have diameters in the range of between about10-35 microns and more typically 17-19 microns. Carbon fibers typicallyhave diameters in the range of between about 5-10 microns and typically7 microns, the fibers (fiberglass and carbon) are not limited to theseranges.

Non-limiting examples of suitable non-glass fiberizable inorganicmaterials include ceramic materials such as silicon carbide, carbon,graphite, mullite, basalt, aluminum oxide and piezoelectric ceramicmaterials. Non-limiting examples of suitable fiberizable organicmaterials include cotton, cellulose, natural rubber, flax, ramie, hemp,sisal and wool. Non-limiting examples of suitable fiberizable organicpolymeric materials include those formed from polyamides (such as nylonand aramids), thermoplastic polyesters (such as polyethyleneterephthalate and polybutylene terephthalate), acrylics (such aspolyacrylonitriles), polyolefins, polyurethanes and vinyl polymers (suchas polyvinyl alcohol).

In one embodiment, the fibers 110 preferably have a high strength toweight ratio. Preferably, the fibers 110 have strength to weight ratioof at least 0.7 GPa/g/cm³ as measured by standard fiber properties at23° C. and a modulus of at least 69 GPa.

Textiles or other assemblies of the infusible, unidirectional fabric 10can be further processed to create composite preforms. One example wouldbe to wrap the fabric 10 around foam strips or other shapes to createthree dimensional structures. These intermediate structures can then beformed into composite structures 400 by the addition of resin in atleast a portion of the void space 120 in the fabric 10.

The infusible, unidirectional fabric 10 can be further processed into aninfused, unidirectional composite 400 as illustrated in FIG. 2 with theaddition of resin in at least a portion of the void space 120 in thefabric 10, preferably filling up approximately all of the void spacewithin the fabric 10.

The infusible, unidirectional fabric 10 is impregnated or infused with aresin 300 which flows, preferably under differential pressure, throughthe fabric 10 at least partially filling the void space creating theinfused, unidirectional composite 400. The infused, unidirectionalcomposite 400 could also be created by other wetting or compositelaminating processes including but not limited to hand lay-up, filamentwinding, and pultrusion. Preferably, the resin flows throughout theinfusible, unidirectional fabric 10 (and all of the other reinforcingmaterials such as reinforcing sheets, skins, optional stabilizinglayers, and strips) and cures to form a rigid, composite 400.

It is within the scope of the present invention to any type ofhardenable resin to infuse or impregnate the porous and fibrousreinforcements of the cores and skins. Thermoset resins, such asunsaturated polyester, vinyl ester, epoxy, polyurethane, acrylic resin,and phenolic, are liquid resins which harden by a process of chemicalcuring, or cross-linking, which takes place during the molding process.Thermoplastic resins, such as polyethylene, polypropylene, PET and PEEK,are liquefied by the application of heat prior to infusing thereinforcements and re-harden as they cool within the panel. In oneembodiment, the resin 300 is an unsaturated polyester, a vinylester, anepoxy resin, a polyurethane resin, a bismaleimide resin, a phenol resin,a melamine resin, a silicone resin, or thermoplastic PBT or Nylon ormixtures thereof. Unsaturated polyester and epoxy are preferred due totheir moderate cost, good mechanical properties, good working time, andcure characteristics.

In some commercial applications, the epoxy based resins have higherperformance (fatigue, tensile strength and strain at failure) thanpolyester based resins, but also have a higher cost. The uniformlyspaced fibers in the fabric 10 may increase the performance of acomposite 400 using an unsaturated polyester resin to levels similar tothe performance levels of the epoxy resin composite, but with a lowercost than the epoxy resin system.

Having the resin 300 flow throughout the infusible, unidirectionalfabric 10 under differential pressure may be accomplished by processessuch as vacuum bag molding, resin transfer molding or vacuum assistedresin transfer molding (VARTM). In VARTM molding, the components of thecomposite are sealed in an airtight mold commonly having one flexiblemold face, and air is evacuated from the mold, which applies atmosphericpressure through the flexible face to conform the composite 400 to themold. Catalyzed resin is drawn by the vacuum into the mold, generallythrough a resin distribution medium or network of channels provided onthe surface of the panel, and is allowed to cure. Additional fibers orlayers such as surface flow media can also be added to the composite tohelp facilitate the infusion of resin. A series of thick yarns such asheavy rovings or monofilaments can be spaced equally apart in one ormore axis of the reinforcement to tune the resin infusion rate of thecomposite.

As an alternate to infusion of the infusible, unidirectional fabric 10with liquid resin, the fabric may be further pre-impregnated(pre-pregged) with partially cured thermoset resins, thermoplasticresins, or intermingled with thermoplastic fibers which are subsequentlycured (or melted and solidified) by the application of heat.

The infused, unidirectional composite 400 may be used as a structure orthe composite 400 have additional processes performed to it or haveadditional elements added to form it into a structure. It may also bebonded to other materials to create a structure including incorporationinto a sandwich panel. In one embodiment, skin sheet materials such assteel, aluminum, plywood or fiberglass reinforced polymer may be addedto a surface of the composite 400. This may be achieved by adding theadditional reinforcement layers while the resin cures or by adhesives.Examples of structures the composite may be (or be part of) include butare not limited to wind turbine blades, boat hulls and decks, rail cars,bridge decks, pipe, tanks, reinforced truck floors, pilings, fenders,docks, reinforced beams, retrofitted concrete structures, aircraftstructures, reinforced extrusions or injection moldings or other likestructural parts. In many of the above mentioned structures, fatiguelife is an important consideration. The infused, unidirectionalcomposite 400 may improve the fatigue performance of these structuralparts.

Composites incorporating a bridged network of unidirectional fibers 100can realize higher fiber volume fractions compared to those made withconventional reinforcements. Higher fiber volume fractions increase themodulus and strength of the composites, particularly in the direction ofthe fiber axis. The uniformity of fiber distribution and lack of fibercrimp due to stitching or off-axis fibers enables higher compressionstrength and enhanced fatigue durability. Composites with thesecharacteristics are also resistant to delamination and therefore providesignificant damage tolerance. These benefits can allow for longer,lighter, more durable and/or lower cost structures in numerousapplications including wind turbine blades.

One benefit of the fabric with infusible uniformly spaced fibers is theopportunity to utilize the fabric in specific subsections of thestructure where the demonstrated performance benefit is most applicable.

Wind turbine blades are an example of a large composite structure thatcan benefit from use of infusible, unidirectional fabrics in specificareas. The loading patterns on wind turbine blades are complex, and thestructure is designed to satisfy a range of load requirements. Forexample, wind turbine blades are designed using at least four differentdesign criteria. The blade must be stiff enough to not strike theturbine tower, strong enough to withstand the maximum expected wind gustloads, durable enough to tolerate hundreds of millions of cycles due tothe rotation of the generator, and sufficiently resistant to buckling toavoid collapsing when flexed under the combined stress induced by theblade itself and the wind loads.

FIG. 7 is a schematic of a wind turbine 1700 which contains a tower1702, a nacelle 1704 connected to the top of the tower, and a rotor 1706attached to the nacelle. The rotor contains a rotating hub 1708protruding from one side of the nacelle, and wind turbine blades 1710attached to the rotating hub.

FIG. 8 is a schematic of a wind turbine blade 1710. The blade representsa type of airfoil for converting wind into mechanical motion. Theairfoil 1800 extends from a root section 1802 at one end along alongitudinal axis to the tip section 1804 at the opposing end.

Sectional view A-A in FIG. 9 from FIG. 8 shows a typical blade crosssection and identifies four functional regions around the perimeter ofthe wind turbine blade air foil. The leading edge 1806 and trailing edge1808 are the regions at the ends of the line extending along the maximumchord width W. The leading and trailing edge regions are connected bytwo portions of a blade shell, a suction side shell 1810 and a pressureside shell 1812. The blade shells are connected via a shear web 1814which helps stabilize the cross section of the blade during service.

The blade shells generally consist of one or more reinforcing layers1816 and may include core materials 1818 between the reinforcing layersfor increased stiffness.

FIG. 9 also identifies two primary structural elements or spar caps 820located within both the pressure side and suction side shell regionswhich both extend along the longitudinal axis of the blade as shown inFIGS. 10 and 11. FIG. 10 represents a plan view of a blade as viewedfrom either the pressure side or suction side of the blade while FIG. 10is the sectional view B-B as illustrated in FIG. 8. FIG. 9 alsoidentifies a leading edge spar 1822 structural element within theleading edge region, and an additional trailing edge spar 1824structural element within the trailing edge region. FIG. 12 is a viewalong the length of the blade showing a piece of the blade shell withvarious layers.

During the wind turbine blade design process, different sections of thestructure are optimized based on the most critical design criteria forthat section. For example, in blades using fiberglass reinforced sparcaps, the size of the spar caps can be based on the stiffnessrequirements to avoid hitting the turbine tower or the fatiguerequirements over which the spar cap can be expected to remain intactover hundreds of millions of load cycles. The nature of the designprocess and the requirements imposed on the various sections of theblade can benefit from materials which offer the opportunity to bedeployed locally within that section. A spar cap reinforcement materialwith improved fatigue resistance could allow more optimized wind turbineblades when fatigue performance dictates the size and weight of the sparcaps.

The infusible, unidirectional fabric 10 may be formed by any suitablemanufacturing method. One method to form the infusible, unidirectionalfabric begins with forming the fabric, or fiber tows. The fabriccontains a plurality of fibers and void space between the fibers.Preferably the fabric then goes through one or multiple fiber towspreading devices, which spread a fiber bundle into a fabric sometimesin the form of a fiber tape or fiber band. This step can break thebinder which has already existed in the fiber bundle and re-distributefiber space more uniformly. The tow spreading device can be any suitabledesign. In one preferred embodiment the tow spreading device(s)comprising several football-shaped rolls, and the fabric is spread whenit is pushed against the football shaped rolls. In another embodiment,the fabric is spread by blowing air to the bundle. In anotherembodiment, the fabric is spread by immersing into water and nippedunder pressure.

After spreading, preferably the fabric is then combined with otherspread bundles of fibers in the fiber direction to form a heavier orwider unidirectional fiber tape, fiber sheet, fiber band, or fabric. Inone embodiment, two 9600 Tex (Tex is a unit of measure for the linearmass density of fibers and is defined as the mass in grams per 1000meters) bundles of fibers are spread independently and then combinedtogether to form a 25.4 mm wide fabric or tape. In one embodiment, eight9600 Tex bundles of fibers are combined together to form anapproximately 500 gsm, 150 mm wide fabric. In another embodiment,multiple 9600 Tex bundles of fibers are combined together to form anapproximately 1000 gsm, 400 mm wide fabric. In another embodiment,multiple 4800 Tex bundles of fibers are combined together to form theunidirectional fabric.

The fabric (in the form of a fiber tape, fiber band or fabric) is thencoated with a coating liquid that contains the bridge polymer or thechemicals that can react and make the bridge polymer. In one embodiment,the polymer bridges are formed beginning with a polymer in waterdispersion or polymer water solution. Preferably the polymer in waterdispersion is an emulsion. The emulsion contains both a continuoussolvent phase and a discontinuous dispersed liquid phase. The two phasesare chosen so that the discontinuous dispersed phase is sufficientlystable that it does not agglomerate or solidify on the time scalerequired for emulsion preparation and coating at typical emulsionpreparation and coating temperatures. This typically requires the resinto be stable for a period of at least several minutes. SBR latex orwater borne polyurethane are preferred due to their moderate cost, goodmechanical properties, good adhesion to fibers. In one embodiment, theaverage size of the particles in the dispersed phase (called dispersedparticles or micelles or referred to as the discontinuous phase) in theemulsion is less than 50 μm, preferably less than 10 μm. These dispersedparticles make up at least about 0.5% by weight of the emulsion, morepreferably at least about 1% by weight, more preferably at least about3% by weight. In another embodiment, the emulsion contains between about3 and 10% by weight of dispersed particles. The continuous phase of theemulsion can contain an aqueous, a non-aqueous liquid, or a mixture ofboth. Preferably the solvent is aqueous or polar because of the cost andenvironmental concerns, wettability of the fiber, flammability issuesand ability to create an emulsion with the dispersed phase. The solventmay also contain a surfactant, which may improve the stability of thedispersed phase after emulsification or may make emulsification a morereliable and efficient process.

In one embodiment, it is preferred to add film-forming preventing agentsin to the polymer water dispersion or polymer water solution, becausethe film-forming preventing agents are able to create void space andchannels between fibers by preventing the polymer forming a continuousfilm.

In one preferred embodiment, the film-forming preventing agent is awater soluble material, which can phase separate from the polymer andform solid or liquid phase during water evaporation. Preferably, thewater soluble materials do not make the polymer in water dispersion orpolymer water solution unstable. Sugar (solid or in liquid form) orother water soluble non-ionic materials are preferred. After waterevaporation and polymer solidified, this type of film forming preventingagent will typically be removed from the fabric, leaving voids andchannels in the fabric.

In one embodiment, sugar is used as the film-forming preventing agent.In this embodiment, the polymer solid content is between about 1% and60% of the polymer in water dispersion or polymer in water solution.More preferably the polymer solid content is between about 3% and 20%.The sugar to polymer solid content ratio by weight is between about0.5:1 and 10:1, more preferably between 1:1 and 5:1. Too little sugarmay not prevent the polymer from forming films and may not create enoughvoid space and channels in the fabric; too much sugar may make thepolymer bridges weak.

In another embodiment, glycerin is used as the film-forming preventingagent. In this embodiment, the polymer solid content is between about 1%and 60% of the polymer in water dispersion or polymer in water solution.More preferably the polymer solid content is between about 3% and 20%.The glycerin to polymer solid content ratio by weight is between about0.5:1 and 20:1, more preferably between 1:1 and 10:1. Too littleglycerin may not prevent the polymer from forming films and may notcreate enough void space and channels in the fabric; too much may makethe polymer bridges weak.

The polymer in water dispersion or polymer in water solution may beapplied to the fiber bundles by any suitable coating method that resultsin the coating liquid filling the void spaces between the fibers andwetting the surface of the fibers. The fiber tape, fiber band or fabricis then treated to cause solidification of the bridge polymer andforming phase separation between bridge polymer and this type of filmforming preventing agent. The bridge polymer chemical(s) can solidify byundergoing chemical reaction, cooling below its(their) melt point,precipitating, crystallizing, or evaporation of a portion of themixture. In one preferred embodiment, this phase change occurs becauseof evaporation of water. In another preferred embodiment, this phasechange occurs because of a chemical reaction, such as polymerization orcrosslinking of mixture that may contain monomers, oligomers,cross-linkers, and initiators; these are commonly available asthermosetting resins that are paired with either a hardener orinitiator. The liquid may also contain catalysts which may affect therate of solidification of the polymer. It may also contain othersolvents that affect the stability of emulsion, the rate ofsolidification. After the bridge polymer rich phase has solidified, thefiber tape, fiber band or fabric is treated to remove the film formingpreventing agent phase and leave an infusible, unidirectional fibertape, fiber band or fabric.

In another preferred embodiment, the film-forming preventing agents area combination of blowing agents and frothing agents (or foaming agents).When the combination of blowing agents and frothing agents are used,preferably a gelling agent is also added to stabilize the polymer foam.The blowing agents may be any suitable materials that can generate smallair bubbles when exposed to a stimulus after coating the polymer inwater dispersion or polymer in water solution onto the fabric. Frothingagents or foaming agent in the coating liquid help stabilize the airbubbles, making the bubbles stable for longer periods of time and alsoallowing them to grow bigger (with the help from the blowing agents).During or after the foaming stage, bridge polymer chemical(s) start tosolidify by undergoing chemical reaction, cooling below its melt point,precipitating, crystallizing, or evaporation of a portion of themixture. In one preferred embodiment, this phase change occurs becauseof evaporation of water. In another preferred embodiment, this phasechange occurs because of a chemical reaction, such as polymerization orcrosslinking of mixture that may contain monomers, oligomers,cross-linkers, and initiators; these are commonly available asthermosetting resins that are paired with either a hardener orinitiator. The liquid may also contain catalysts which may affect therate of solidification of the polymer. It may also contain othersolvents that affect the stability of emulsion, the rate ofsolidification, the structure of the resulting bridges, or the surfaceof the bridges. The gelling agent can increase the viscosity of theliquid, transfer the solvent from liquid state to a gel state. It canhelp to further stabilize the bubbles and polymer foam, and locks thephase structure of the coating material during the polymer solidifystep.

In one embodiment, the blowing agent is water. Water can quicklyevaporate under heat and creates bubbles. In another embodiment, theblowing agent is carbon dioxide that has dissolved in water. In anotherembodiment, the blowing agent is low boiling point organic liquid. Thefrothing agents or foaming agents include but not limited to ionicsurfactant such as sodium dodecyl sulfate (SDS), sodiumdodecylbenzenesulfonate (NaDDBS), or non-ionic block copolymer such asethylene oxide and propylene oxide copolymer. One example of the blockcopolymer is Pluronic® from BASF. A gelling agent is also preferred tobe added to stabilize the polymer foam. The gelling agent includes butnot limited to acacia, alginic acid, bentonite, carbomers,carboxymethylcellulose. ethylcellulose, gelatin, hydroxyethylcellulose,hydroxypropyl cellulose, magnesium aluminum silicate (Veegum®),methylcellulose, Pluronics®, polyvinyl alcohol, sodium alginate,tragacanth, and xanthan gum. A gelling agent with lower criticalsolution temperature (LCST) is preferred because it is soluble in coldwater and gels in hot water. One example of the gelling agent with LCSTcharacteristic is Pluronics® F-127. In one embodiment, foaming agentsand gelling agents are used as the film-forming preventing agent. Thepolymer solid content is between about 1% and 60% of the polymer inwater dispersion or polymer in water solution. More preferably thepolymer solid content is between about 3% and 20%. The frothing agent isbetween about 0.1% and 20% of the total weight, more preferably betweenabout 1% and 10% of the total weight. The gelling agent is between about0.1% and 40% of the total weight, more preferably between 1% and 10% ofthe total weight. In one embodiment, Pluronics® F-127 is used as afoaming agent and also a gelling agent, preferably between 1% and 15% byweight in the coating mixture, more preferably between 3% and 10% byweight in the coating mixture.

The coating mixture with the blowing agent, frothing agent (or foamingagent) and gelling agent can be applied to the fiber tape, fiber band orfabric through many coating methods that are typically used to applycoating mixture to fiber bundles or fabrics. The emulsion can be appliedusing dip, nip, roll, kiss transfer, spray, slot, slide, die, curtain,or knife coating processes among others. The coating should be appliedso that it fills the void spaces within the fiber bundles and so that itdoes not destabilize the coating mixture during the coating process.Mechanical action, such as passing over a series of rollers, passingover a roller with a patterned surface, pumping the emulsion through thefiber bundles, repeated saturation of the bundles with the emulsion,sonication or oscillating the fiber bundle tension may aid inhomogeneously filling the void spaces between fibers within the fiberbundle. The amount of applied coating mixture can be metered usingroutinely practiced metering methods available for the aforementionedcoating methods.

After coating the fabric, the blowing agent is activated by exposing toa stimulus to generate bubbles. In one preferred embodiment, water isused as the blowing agent. The coated fiber tape, fiber band or fabricis exposed to heat, resulting rapid vaporization of water and bubbleformation in water. Preferably the wet fibers are directly contact on ahot surface. Preferably the temperature of the hot surface is at least100° C., more preferably the temperature of the hot surface is at least120° C., more preferably the temperature of the hot surface is at least150° C. The bubbles that are generated by the blowing agent arestabilized by the frothing agent or foaming agent, and are furtherstabilized by the gelling agent. In one preferred embodiment, Pluronics®F-127 is used as a foaming agent and also a gelling agent, preferablybetween 1% and 15% by weight in the coating mixture, more preferablybetween 3% and 10% by weight in the coating mixture. During or after theactivating the blowing agent, the chemicals in the coating mixture issolidified to form the bridge structure. This bridge forming process hasbeen shown to impact the formation of the bridge structure. An importantpart of the bridge forming process is to allow enough heat to transferto the blowing agent rapidly to generate enough bubble before polymerhas had time to solidify. And another important part of the bridgeforming process is to stabilize the foam during polymer solidification.The size of the void space or channel in the fiber tape, fiber band orfabric, is controlled by the concentration of the polymer in the coatingmixture, the concentration of the film-forming preventing agents, andthe method to dry the fibers. The more effective of the activating theblowing agent, the better bridge structure can be made. The foamingagent and gelling agent is critical to prevent the polymer forming afilm. If the blowing agent is not functioning well before the polymerhas solidified, the polymer will want to form a continuous film on andbetween fibers. If the foam structure is not stable enough and breaksbefore the polymer has solidified, the polymer will also want to form acontinuous film on and between fibers.

Likewise, if the water is removed from the system before the polymer hassolidified, the polymer will want to spread out onto the functionalizedglass fibers. This favorable surface interaction will cause the polymerto form films on and between the fibers, greatly reducing the ability ofthe fabric to be infused into a composite material using standard resininfusion techniques.

During or after the discontinuous phase has solidified, the coatedfabric may be dried to remove the residual solvent. The drying processhas been shown to impact the performance of the infusible,unidirectional fabric in composite. To increase the production rate itis preferable to dry the fiber bundles at a temperature above roomtemperature, preferably at or above the boiling point of the solvent,provided that the drying temperature and time are below a temperatureand time combination that causes the structure of the bridges to change,for example by decomposing the material forming the bridges, causingthem to flow, or causing the bridges to become significantly lessfatigue resistant.

In one embodiment, the coated fabric is dried at a temperature betweenabout 80 and 150° C. for a time of between about 3 and 60 minutes. Inone particular embodiment, the coated fabric is dried at temperature of150° C. for 3 minutes. In another embodiment, the surface temperature offiber bundles immediately after drying is at least 110° C. The energyimparted to the fabric is sufficient to remove at least 90% of thesolvent by weight, preferably at least 99.7% by weight. After drying inone embodiment, the solvent content in the fabric is preferably lessthan 1% by weight, more preferably less than about 0.1% by weight.

Mechanical action may also be used during various steps of production.Mechanical action may be used only once in the process, or many timesduring different steps of the process. Mechanical action may be in theform of sonication, wrapping the fabric around a roller under tension,moving the fabric normal to its uniaxial or machine direction in thecoating bath, compressing/relaxing fabric, increasing or reducing thetension of the fabric, passing it through a nip, pumping the coatingliquor through the fabric, using rollers in the process with surfacepatterns. These surface patterns can have similar characteristicdimensions to the diameter of the fiber, the outside diameter of thefiber bundle, or the width of the fabric. It has been found that theaddition of mechanical action during production of the infusible,unidirectional fabric may temporarily increase or decrease the spacebetween fibers either once or multiple times, provide a pressuregradient to increase flow of the emulsion or suspension into, throughoutand out of the bundle, and homogenize the distribution of dispersedpolymer phase within the bundle. In one embodiment, the coated fabric issubjected to mechanical action after the coating step. In anotherembodiment, the coated fabric is subjected to mechanical action duringthe drying step. In another embodiment, the coated fabric is subjectedto mechanical action after the drying step. The mechanical action mayhelp to soften the fabric and create additional discontinuity in thecoating by breaking large polymer bridges into smaller pieces.

After the infusible, unidirectional fabric is formed, it may be furtherprocessed into a bridged composite using the infusing the infusible,unidirectional fabric with resin as described previously.

EXAMPLES

The invention will now be described with reference to the followingnon-limiting examples, in which all parts and percentages are by weightunless otherwise indicated.

Fatigue Testing Method

During testing, fatigue loads are normally characterized by an R valuewhich is defined as the ratio of minimum to maximum applied stress. Byconvention, compressive stress is taken to be a negative number andtension stress is taken as a positive number. Full characterization offatigue performance involves testing over a range of R values such asR=0.1, −1, and 10, which corresponds to tension-tension,tension-compression, and compression-compression fatigue cyclesrespectively. Tension-tension fatigue with R=0.1 is a key metric offatigue performance and was used to quantify the fatigue behavior ofcomposite systems herein.

The fatigue performance of the composite materials made with thefusible, unidirectional fabric was measured using a standardtension-tension fatigue test. After composite panels were infused,composite tabs (1.6 mm thick) were bonded to the surfaces of the panelsin the appropriate locations to establish the specimen gage length.Specimen details and dimensions were similar to those specified in ISO527-5 with straight sided specimens 25 mm wide.

The specimens were environmentally conditioned for 40 hours at 23°C.+/−3° C. and 50%+/−10% relative humidity.

Using a servohydraulic test machine equipped with hydraulic wedge grips,the specimens were gripped using the minimum pressure required to avoidslipping. The machine was programmed to load the specimen in sinusoidalfashion using a specified frequency, mean load, and load amplitude.Cyclic loading continued until the specimen failed.

Typical schemes employ testing at a given R value with peak stressvalues chosen for the different tests of 80%, 60%, 40%, and 20% of thequasi-static strength. Test frequency is chosen to accelerate testingwhile ensuring the specimen temperature does not increase significantly(less than 35° C. for room temperature testing). This means that lowerstress level testing can be done at higher frequencies than higherstress level tests.

The output of a typical fatigue testing regimen at a given R value isknown as an S-N curve which relates the number of cycles a material cansurvive to specified loading conditions. S-N curves provide the mostcommon comparison tool for basic fatigue performance evaluation. S-Ncurves for well-defined conditions are frequently used to compare thefatigue performance of different composite systems under similarloading. Improvement in R=0.1 fatigue testing, generally indicates asignificant change in the fatigue behavior of a composite material.

Wind blades are generally designed to withstand over 10⁸ loading andunloading cycles, however testing materials to such extremes is animpractical exercise. Comparisons are often made among materials atintermediate points such as the one million or 10⁶ cycle performance. Inorder to screen samples, a specific peak loading level of 800 N/mm ofspecimen gage section width was applied with an R value of 0.1(tension-tension fatigue) and the number of cycles to failure wasmeasured for each sample. This loading was chosen to balance the amountof time required to perform an experiment with the reliability of thedata for predicting fatigue performance at more typical levels ofstrain. The same loading levels of 800 N/mm were also applied to acontrol composite samples made from traditional reinforcing fabrics.

Sample Layup Procedure

The layup procedure was to stack the layers on top of a flat glass toolprepared with a mold release and covered with one layer of releasefabric (peel ply). A laser crosshair was used to provide a fixedreference for alignment of the fibers in each layer. Both pieces offabric were placed so that the fibers on the top surfaces ran in thesame direction. Then a 900 layer of the unidirectional fabric wasaligned with the crosshair and placed with the unidirectional tows up.This was followed with a 090 layer of unidirectional fabric that wasaligned and placed with the unidirectional side down. The next 900 layerof unidirectional fabric was placed with the unidirectional tows up anda final 090 layer was placed with the unidirectional tows facing down.The last two layers of ±45 fabric were placed so that the fibers ontheir top surface ran perpendicular to the fibers on the top surface ofthe ±45 fabric on the bottom two layers of the fabric stack. Finally,the laminate stack was covered with another layer of release fabric(peel ply).

The vacuum infusion molding process was used to impregnate the laminateswith resin. On top of the release fabric for each laminate, a layer offlow media was used to facilitate resin flowing into the reinforcementplies. The entire laminate was covered with a vacuum bagging film whichwas sealed around the perimeter of the glass mold. Vacuum was applied tothe laminate and air was evacuated from the system. Resin was thenprepared and pulled into the reinforcement stack under vacuum untilcomplete impregnation occurred. After the resin was cured, the compositepanel was removed from the mold and placed in an oven for post-curing.

Example 1

An unsaturated polyester control sample was made using the sample layupprocedure using the 090 fabric and the ±45 fabric. The stacked textileswere infused in a standard vacuum infusion apparatus at a vacuum of lessthan 50 mbar with unsaturated polyester resin (Aropol Q67700 availablefrom Ashland) and 1.5 parts per hundred resin (phr) methyl ethyl ketoneperoxide (MEKP). The resin flow direction was along the 0° direction ofthe 090 fabric. The panel was cured at room temperature for more than 8hours and further post cured at 80° C. for more than 4 hours. Fatiguetesting of the unmodified glass reinforced unsaturated polyestercomposite at R=0.1 with a load of 1450 N/mm of specimen gage sectionwidth measured a lifetime of approximately 1×10⁴ cycles.

Example 2 to Example 7 showed how the film forming preventing agentsaffect the infusibility of the fiber fabric. The fiberglass fabrics usedin Examples 2-6, and 8 were in small widths so will be referred toherein as fiberglass tapes.

Example 2

A fiberglass tape was made in the following manner. First, a 9600 Texfiberglass tow from PPG (HYBON® 2026) was spread into a 20 mm wide tapeby a fiber tow spreading device. Next, four of the 20 mm wide tapes werecombined and aligned in the same direction to form a 40 mm wide tapewith twice the original tape thickness. A SBR latex (GENCAL® 7555 fromOMNOVA) was mixed with water at a SBR latex to deionized water ratio of1:4. The fiber tape was then dipped in the coating mixture and dried inan oven at 150° C. for 30 minutes. Next, the fiber tape was washed usingdeionized water and dried in oven at 150° C. for 15 minutes.

Example 3

A 40 mm wide, fiberglass tape was made using the same fiberglassmaterials and process as Example 2. A SBR latex (GENCAL® 7555 fromOMNOVA) was mixed with water and glycerin at a SBR latex to deionizedwater to glycerin ratio of 1:2:2. The fiber tape was dipped in thecoating mixture and dried in an oven at 150° C. for 30 minutes. Next,the fiber tape was washed by deionized water and dried in oven at 150°C. for 15 minutes.

Example 4

A 40 mm wide, fiberglass tape was made using the same fiberglassmaterials and process as Example 2. A SBR latex (GENCAL® 7555 fromOMNOVA) was mixed with glycerin at a SBR latex to glycerin ratio of 1:4.The fiber tape was dipped in the coating mixture and dried in an oven at150° C. for 30 minutes. Next, the fiber tape was washed by deionizedwater and dried in oven at 150° C. for 15 minutes.

Example 5

A 40 mm wide, fiberglass tape was made using the same fiberglassmaterials and process as Example 2. A SBR latex (GENCAL® 7555 fromOMNOVA) was mixed with water and glycerin at a SBR latex to water toglycerin ratio of 1:1:8. The fiber tape was then dipped in the coatingmixture and dried in an oven at 150° C. for 30 minutes. Next, the fibertape was washed by deionized water and dried in oven at 150° C. for 15minutes.

Example 6

A 40 mm wide, fiberglass tape was made using the same fiberglassmaterials and process as Example 2. A waterborne polyurethane (SYNTEGRA®YM 2000 from Dow Chemical) was mixed with water at a YM 2000 todeionized water ratio of 1:6. The fiber tape was then dipped in thecoating mixture and dried in an oven at 80° C. for 4 hours. Next, thefiber tape was washed by deionized water and dried in oven at 80° C. for12 hours.

Example 7

A fiberglass tape was made in the following manner. First, a 4800 Texfiberglass tow from PPG (HYBON® 2002) was wrapped on a piece of plasticto form a roughly 1000 gsm fabric. A waterborne polyurethane (SYNTEGRA®YM 2000 from Dow Chemical) was mixed with sugar and water at a YM 2000to sugar to deionized water ratio of 1:2.7:6. The fiber tape was thendipped in the coating mixture and dried in an oven at 80° C. for 4hours. Next, the fiber tape was washed by deionized water for 2 days anddried in oven at 80° C. for 12 hours.

The infusibility of the fiber tape for Examples 2-7 were characterizedin the following manner: Several water droplets with 0.01% water solublecolor dye Acid Blue 9 were dropped on the center surface of fiber fabricby using a 5 mL transfer pipette, and the time that how long it took forthe droplets to completely infuse into the fabric was used as anindication of infusibility of the fiber tape. In this method,“completely infuse into the fiber fabric” means that more than 99% waterfrom original droplets is staying between the upper inner surface andlower inner surface of the fabric.

For the fiber tape in Example 2, the droplets stayed on the surface ofthe tape and cannot infuse into the tape. For the tape in Example 3, thedroplets took about several seconds to infuse into the tape. For thetape in Example 4 and 5, the droplets immediately infused into the tape.The differences between these three examples showed how the film formingpreventing agent (glycerin in Example 2, 3 and 4) affected theinfusibility of the final article.

For the fiber tape in Example 6, the droplets stayed on the surface ofthe tape and did not infuse into the tape. For the tape in Example 7,the droplets took about half minute to infuse into the tape. Thedifferences between these two examples showed how the film formingpreventing agent (sugar in Example 6) affected the infusibility of thefinal article.

An optical microscope image of the SBR coating in Example 5 was takenand determined that most of the bridges had a bridge width narrower than60 microns.

Example 8

A fiberglass tape was made in the following manner. A waterbornepolyurethane (SYNTEGRA YM 2000), a blocked isocyanate basedcross-linking agent (Milliken MRX), sugar and water were mixed at a massratio of 103:5.6:277:620. A fiber roving (PPG HYBON® 2002) was spread toform a roughly 500 gsm fabric (sometimes referred to as a tape as it ishas a low width). Then the fiber tape was dipped into the coatingmixture. The fabric was dried at 80° C. for 4 hours and washed by waterfor 12 hours. Next, the fiber tape was dried in oven at 80° C. for 12hours.

The FIG. 3 shows an SEM image of the cross section of the fabric. Onecan see the polymer bridges connecting fibers.

Example 9

A fiberglass fabric was made in the following manner. 7.6 g waterbornepolyurethane (BONDTHANE J-884-A from Bond Polymers International), 0.2 gcrosslinking agent (Milliken MRX), 9 g sugar and 100 g water was mixedto made the coating solution. A total mass of about 150 g fiber rovings(HYBON® 2002) from PPG were uniformly fixed on an 8″ by 24″ plate byholing ends of rovings under tension. A piece of SPUNFAB lightweightadhesive web was put on top of the rovings. The coating mixture waspulled onto the rovings and a rubber roller was used to apply uniformcoating and nip the excess liquid. The 8″ by 24″ plate together withrovings was dried at 80° C. overnight. Next, the whole fabric wasremoved from the plate and immersed in water for 24 hours and then driedat 80° C.

Example 10

A stack of textiles was formed in order: Two (2) layers of the infusiblefabric of Example 9, the fibers in the two layers were parallel. Theside having SPUNFAB web was on the outer side. The stacked textiles wereinfused in a standard vacuum infusion apparatus at a vacuum of −25 in.Hg (about 169 mbar) with 98.77% wt unsaturated polyester resin (AropolG300 available from Ashland) and 1.33% wt methyl ethyl ketone peroxide(MEKP 925H available from Norox). The resin flow direction was along thefibers. The panel was cured at room temperature more than 8 hours andfurther post cured at 80° C. for more than 4 hours forming thecomposite. FIG. 4 shows a cross section view of the composite. One cansee that the fiber location distribution is more uniform than Example 1.The tensile modulus of the composite is 5% higher than Example 1 whichcomprises traditional stitched unidirectional fabric. The peak stressand peak strain in static tensile test of the composite is about 20%higher than Example 1.

Example 11

A fiberglass fabric was made in the following manner. 8 g waterbornepolyurethane (SYNTEGRA YM 2000 from Dow Chemical), 0.5 g crosslinkingagent (Milliken MRX), 13.5 g sugar and 150 g water was mixed to made thecoating solution. A total mass of about 260 g fiber rovings (HYBON®2002) from PPG were uniformly fixed on an 14″ by 24″ plate by holingends of rovings under tension. A piece of SPUNFAB lightweight adhesiveweb was put on top of the rovings. Another total mass of about 260 gfiber rovings (HYBON® 2002) from PPG were uniformly put on top of theSPUNFAB lightweight adhesive web and fixed on the same 14″ by 24″ plateby holding ends of rovings under tension. In both layers, all fibers arein the same direction. The coating mixture was pulled onto the rovingsand a rubber roller was used to apply uniform coating and nip the excessliquid. And then the whole 14″ by 24″ plate together with rovings wasdried at 80° C. overnight. Next, the whole fabric was removed from theplate, immersed in water for 24 hours, and then dried at 80° C.

Example 12

The fabric in Example 11 was infused in a standard vacuum infusionapparatus at a vacuum of 25 in. Hg (about 169 mbar) with 98.77% wtunsaturated polyester resin (Aropol G300 available from Ashland) and1.33% wt methyl ethyl ketone peroxide (MEKP 925H available from Norox).The resin flow direction was along the fibers. The panel was cured atroom temperature more than 8 hours and further post cured at 80° C. formore than 4 hours. This formed the composite. The tensile modulus of thecomposite is 5% higher than Example 1 which comprises traditionalstitched unidirectional fabric. The peak stress and peak strain instatic tensile test of the composite is about 20% higher than Example 1.In the R=0.1 tensile fatigue test, the cycles to failure of thiscomposite is about 12 times of the control which comprises traditionalstitched unidirectional fabric.

All references, including publications, patent applications, andpatents, cited herein are hereby incorporated by reference to the sameextent as if each reference were individually and specifically indicatedto be incorporated by reference and were set forth in its entiretyherein.

The use of the terms “a” and “an” and “the” and similar referents in thecontext of describing the invention (especially in the context of thefollowing claims) are to be construed to cover both the singular and theplural, unless otherwise indicated herein or clearly contradicted bycontext. The terms “comprising,” “having,” “including,” and “containing”are to be construed as open-ended terms (i.e., meaning “including, butnot limited to,”) unless otherwise noted. Recitation of ranges of valuesherein are merely intended to serve as a shorthand method of referringindividually to each separate value falling within the range, unlessotherwise indicated herein, and each separate value is incorporated intothe specification as if it were individually recited herein. All methodsdescribed herein can be performed in any suitable order unless otherwiseindicated herein or otherwise clearly contradicted by context. The useof any and all examples, or exemplary language (e.g., “such as”)provided herein, is intended merely to better illuminate the inventionand does not pose a limitation on the scope of the invention unlessotherwise claimed. No language in the specification should be construedas indicating any non-claimed element as essential to the practice ofthe invention.

Preferred embodiments of this invention are described herein, includingthe best mode known to the inventors for carrying out the invention.Variations of those preferred embodiments may become apparent to thoseof ordinary skill in the art upon reading the foregoing description. Theinventors expect skilled artisans to employ such variations asappropriate, and the inventors intend for the invention to be practicedotherwise than as specifically described herein. Accordingly, thisinvention includes all modifications and equivalents of the subjectmatter recited in the claims appended hereto as permitted by applicablelaw. Moreover, any combination of the above-described elements in allpossible variations thereof is encompassed by the invention unlessotherwise indicated herein or otherwise clearly contradicted by context.

What is claimed is:
 1. An infusible, unidirectional fabric having anupper inner surface and a lower inner surface comprising: a plurality ofunidirectional fibers having a diameter and a length, wherein theunidirectional fibers are spaced uniformly in the unidirectional fabric;a plurality of bridges, each bridge being connected to at least 2unidirectional fibers and wherein at least 70% by number of fiberscomprise at least one bridge connected thereto forming a bridged networkof unidirectional fibers, wherein the bridges comprise a bridgingpolymer, wherein between the unidirectional fibers the bridges each havea width and a bridge width minimum, and wherein at least 50% by numberof the bridges have a bridge width minimum less than about 2millimeters; and, a plurality of void spaces between the unidirectionalfibers, wherein the void spaces are interconnected, wherein the fabrichas a volume fraction of voids of between about 8 and 70%, and whereinthe fabric has a volume fraction of fibers of between about 35 and 85%.2. The infusible unidirectional fabric of claim 1, wherein the bridgednetwork of unidirectional fibers have a tensile strength of at least 200Pa in the direction perpendicular to the unidirectional fibers.
 3. Theinfusible, unidirectional fabric of claim 1, wherein the bridgingpolymer forms between about 0.1 and 30% of the effective cross-sectionalarea of the infusible, unidirectional fabric.
 4. The infusible,unidirectional fabric of claim 1, wherein the unidirectional fiberscomprise a material selected from the group consisting of glass, carbon,aramid, polyethylene, polyester, polyamide, and mixtures thereof.
 5. Theinfusible, unidirectional fabric of claim 1, wherein the infusible,unidirectional fabric does not comprises any stitching fibers or yarns.6. An infused, unidirectional composite comprising: at least oneunidirectional fabric having an upper inner surface and a lower innersurface, the unidirectional fabric comprising a plurality ofunidirectional fibers having a diameter and a length, wherein theunidirectional fibers are spaced uniformly in the unidirectional fabric;a plurality of bridges, each bridge being connected to at least 2unidirectional fibers and wherein at least 70% by number of fiberscomprise at least one bridge connected thereto forming a bridged networkof unidirectional fibers, wherein the bridges comprise a bridgingpolymer, wherein between the unidirectional fibers the bridges each havea width and a bridge width minimum, and wherein at least 50% by numberof the bridges have a bridge width minimum less than about 2millimeters; and, a cured resin between the unidirectional fibers,wherein the cured resin is continuous through the composite, wherein thecomposite has a volume fraction of cured resin of between about 8 and70%, and wherein the composite has a volume fraction of fibers ofbetween about 35 and 85%.
 7. The infused, unidirectional composite ofclaim 6, wherein the bridging polymer forms between wherein the bridgingpolymer forms between about 0.1 and 30% of the effective cross-sectionalarea of the infused, unidirectional composite.
 8. The infused,unidirectional composite of claim 6, wherein the composite comprises atleast two or more adjacent unidirectional fabrics.
 9. A structurecomprising the infused, unidirectional composite of claim
 6. 10. Thestructure of claim 9, wherein the structure is selected from the groupconsisting of wind turbine blades, bridges, boat hulls, boat decks, railcars, pipes, tanks, reinforced truck floors, pilings, fenders, docks,reinforced wood beams, retrofitted concrete structures, aircraftstructures, reinforced extrusions and injection moldings.
 11. A windturbine blade comprising an infused, unidirectional composite in asection of the wind turbine blade selected from the group consisting ofspar section, a root section, leading edge, trailing edge, wherein theinfused, unidirectional composite comprises: a plurality ofunidirectional fibers having a diameter and a length, wherein theunidirectional fibers are spaced uniformly in the unidirectional fabric;a plurality of bridges, each bridge being connected to at least 2unidirectional fibers and wherein at least 70% by number of fiberscomprise at least one bridge connected thereto forming a bridged networkof unidirectional fibers, wherein the bridges comprise a bridgingpolymer, wherein between the unidirectional fibers the bridges each havea width and a bridge width minimum, and wherein at least 50% by numberof the bridges have a bridge width minimum less than about 2millimeters; and, a cured resin between the unidirectional fibers,wherein the cured resin is continuous through the composite, wherein thecomposite has a volume fraction of cured resin of between about 8 and70%, and wherein the composite has a volume fraction of fibers ofbetween about 35 and 85%.
 12. A process of forming infusible,unidirectional fabric comprising: arranging a plurality ofunidirectional fibers into a unidirectional fabric, wherein theunidirectional fibers are spaced uniformly within the unidirectionalfabric; forming an emulsion or suspension of a solvent, a bridgingpolymer, and a film-forming preventing agent, wherein the bridgingpolymer is dissolvable or dispersible in the solvent, wherein thefilm-forming preventing agent is dissolvable or dispersible; applyingthe emulsion or suspension to the unidirectional fabric; removing thesolvent; removing the film-forming preventing agent to form aninfusible, unidirectional fabric, wherein the infusible, unidirectionalfabric comprises: a plurality of unidirectional fibers having a diameterand a length, wherein the unidirectional fibers are spaced uniformly inthe unidirectional fabric; a plurality of bridges, each bridge beingconnected to at least 2 unidirectional fibers and wherein at least 70%by number of fibers comprise at least one bridge connected theretoforming a bridged network of unidirectional fibers, wherein the bridgescomprise a bridging polymer, wherein between the unidirectional fibersthe bridges each have a width and a bridge width minimum, and wherein atleast 50% by number of the bridges have a bridge width minimum less thanabout 2 millimeters; and, a plurality of void spaces between theunidirectional fibers, wherein the void spaces are interconnected,wherein the fabric has a volume fraction of voids of between about 8 and70%, wherein the fabric has a volume fraction of fibers of between about35 and 85%.
 13. The process of claim 12, wherein the solvent is water.14. The process of claim 12, wherein the film-forming preventing agentis a liquid.
 15. The process of claim 12, further comprising infusingand curing a resin into the infusible, unidirectional fabric.
 16. Theprocess of claim 12, wherein the infusible, unidirectional fabric doesnot comprises any stitching fibers or yarns.
 17. The process of claim12, wherein the bridging polymer forms between about 0.1 and 30% of theeffective cross-sectional area of the infusible, unidirectional fabric.18. A process of forming infusible, unidirectional fabric comprising:arranging a plurality of unidirectional fibers into a unidirectionalfabric, wherein the unidirectional fibers are spaced uniformly withinthe unidirectional fabric; forming an emulsion or suspension of asolvent, a bridging polymer, a blowing agent, a foaming agent and agelling agent, wherein the bridging polymer is dissolvable ordispersible in the solvent; applying the emulsion or suspension to thefabric; activating the blowing agent forming bubbles in the emulsion andsuspension, wherein the foaming agent and gelling agent stabilize thebubbles; removing the solvent forming an infusible, unidirectionalfabric, wherein the infusible, unidirectional fabric comprises: aplurality of unidirectional fibers having a diameter and a length,wherein the unidirectional fibers are spaced uniformly in theunidirectional fabric; a plurality of bridges, each bridge beingconnected to at least 2 unidirectional fibers and wherein at least 70%by number of fibers comprise at least one bridge connected theretoforming a bridged network of unidirectional fibers, wherein the bridgescomprise a bridging polymer, wherein between the unidirectional fibersthe bridges each have a width and a bridge width minimum, and wherein atleast 50% by number of the bridges have a bridge width minimum less thanabout 2 millimeters; and, a plurality of void spaces between theunidirectional fibers, wherein the void spaces are interconnected,wherein the fabric has a volume fraction of voids of between about 8 and70%, wherein the fabric has a volume fraction of fibers of between about35 and 85%.
 19. The process of claim 18, further comprising infusing andcuring a resin into the infusible, unidirectional fabric.
 20. Theprocess of claim 18, wherein the infusible, unidirectional fabric doesnot comprises any stitching fibers or yarns.